The instant invention relates to a valve which limits the output torque of a hydraulic pump driven by an electric motor during start-up of the electric motor to a value which will ensure that the motor can accelerate to its rated speed.
In the instant invention a variable displacement, pressure compensated, hydraulic pump, of the type shown in U.S. Pat. No. 3,250,227 to Kouns and specifically incorporated by reference thereto herein, is driven by an electric motor. The torque output characteristics of some electric motors are such that during start-up the output torque is considerably less than the normal rated running torque of the motor. Some of the causes of low motor torque at start-up are low line voltage, low ambient temperature and high torque demand from the load which the motor is driving. In order to ensure positive acceleration of the electric motor from start-up to its rated speed, the torque required by the driven load, i.e., the hydraulic pump, must be less than the output torque of the electric motor. One method of limiting the required input torque of a variable displacement, hydraulic pump is to reduce the displacement of, i.e., destroke, the pump during start-up of the electric motor. Examples of where variable displacement pumps are destroked on start-up are shown in the following: U.S. Pat. Nos. 2,263,314, 3,834,836 and 4,254,962.
A problem with prior systems which destroke a variable displacement pump during start-up is that they require the use of an auxiliary or external pump. This is undesirable since it increases the cost and weight of the system.
It is desirable to have a device which will destroke a variable displacement pump during start-up of its electric motor which utilizes the output fluid of the variable displacement pump itself to destroke the pump.
It is also desirable to provide a device which will destroke a variable displacement, pressure compensated pump driven by an electric motor and which will automatically relinquish control of the pump to the pressure compensator when the electric motor reaches its rated speed and output torque.
It is further desirable to provide a device which will destroke a variable displacement, hydraulic pump during start-up of its electric motor and will automatically reset itself to assume control of the variable displacement pump each time the electric motor shuts down.